1. Field of the Invention
This invention relates generally to superconducting dynamoelectric machinery, and more specifically, to a dynamoelectric machine having a rotor comprising a superconductive field winding and a ferromagnetic core.
2. Description of the Prior Art
It is known that when certain materials, referred to as superconductors, are cooled to near absolute zero they exhibit a complete loss of electrical resistance. Practical utilization of the zero resistance character of superconductive materials at cryogenic temperatures has recently been applied in dynamoelectric machinery. The development of the intrinsically stable multifilamentary superconductor has made it possible to build stable superconducting windings with relatively high transport current densities in large direct current fields.
The use of the superconductive direct current field winding allows a considerable increase in the field magnetomotive force generated by the windings and greatly increased flux densities in the active air gap of the machine. This increase in flux density obtains considerably increased power density and consequential reductions in the weight and volume of the machine. The size and weight reductions make superconducting machines attractivve for such applications as electric drive ship propulsion systems. Also, higher ratings for turbine generators can be obtained without prohibitive increases in frame size.
It is useful to consider the phenomenon of superconductivity and the related properties of superconductors in order that the present invention may be clearly understood. Superconductivity is the state in which some metals offer no resistance to current and therefore do not generate heat as do normal conductors. The resistance at superconducting temperatures is not merely extremely low, it is exactly zero. Superconductivity occurs only at very low temperatures; the temperature is different for each material and is known as the transition or critical temperature, T.sub.c. At the transition temperature, which is a few degrees above absolute zero, there occurs a thermodynamic transition into the superconducting state. The transition temperature, in the absence of a magnetic field, is approximately 3.7.degree. Kelvin for tin, 7.3.degree. Kelvin for lead, and 8.degree. Kelvin for niobium. For further information on specific properties, see National Bureau of Standards Technical Note 724, "Properties of Selected Superconductive Materials," published by the U.S. Department of Commerce.
In addition to temperature, the strength and geometry of magnetic fields affect superconducting materials. A material will suddenly lose its superconductivity in a high strength magnetic field, even a self-generated field, when it reaches a value known as its critical magnetic field, H.sub.c. There also exists a critical electrical current density, J.sub.c, which is dependent upon both the temperature and the magnetic field. The three parameters T, H, and J define a three dimensional surface separating the superconducting and normal regions as illustrated in FIG. 1 of the drawings. For a given temperature (shaded region of FIG. 1) a superconducting coil will have some design load line as illustrated and an operating point P' chosen to be less than the critical point P, where a normal transition occurs. This return to the normal state is usually called a quench. It should be understood that while the shape of the critical curves for any superconductive material is generally as indicated in FIG. 1, the intercepts at the axes are determined by the properties of the material selected.
Superconductors which are suitable for high current density, high field applications (usually called type II or hard superconductors) are subject to instabilities, where a small disturbance in operating conditions can cause a quench, even though the critical current density, magnetic field, or temperature is not exceeded except in a very small region. The current carrying capability of a single superconductor is limited by the maximum field seen at any point on the conductor. The current rating of a superconductive winding will therefore be greatly reduced by high flux concentration, even in a small region of the winding.
A serious problem involved in superconducting windings is the maintenance of superconductivity under magnetic field conditions which tend to destroy superconductivity. An equally important consideration is that of obtaining the maximum useful external field available from a given amount of superconductive material, once operating stability is achieved.
Various techniques for preventing premature normalization due to non-uniform magnetic field conditions are known in the prior art. One known technique is to divide the superconductor into many fine filaments embedded in a high electrically and thermally conductive material such as high purity copper. The entire conductor is usually twisted about its axis to reduce eddy current losses. The copper dissipates heat from any small portion of the superconductor that may happen to normalize, thus preventing a stray normalization from heating the strand and triggering destruction of the superconductivity throughout the coil. Such a superconductor has been described by M. N. Wilson, et al, in "Experimental and Theoretical Studies of Filamentary Superconducting Composites, part I," "Journal of physics D-Applied physics," November 1970, Vol. 3, p. 1517.
The amount of the copper used in this technique is usually between one and three times the amount of superconductor. Although the use of copper increases operating stability, it has the undesirable effect of significantly reducing the overall current density, particularly when the ratio of copper to superconductor is increased to a proportion greater than 3:1. Thus there exists practical limitations on the use of the copper dissipation technique. Prior art superconducting machines which have utilized iron or other ferromagnetic material in the field structure have done so for reasons not associated with the control of the magnetic field intensity in critical winding regions. For example, U.S. Pat. No. 3,470,396 issued to W. Kafka shows an electric machine having an iron core rotor, the iron core being adapted to produce magnetic forces to compensate for centrifugal forces acting upon the rotor winding. Also, in the copending U.S. application Ser. No. 327,540 by C. J. Mole et al filed Jan. 29, 1973, now a Defensive Publication T917,006 and assigned to the assignee of the present invention, iron or other ferromagnetic material was used in the field structure of a dynamoelectric machine in order to obtain sufficient magnetic flux at those positions required during asynchronous modes of operation.
A superconductive field winding for the rotor of a dynamoelectric machine which utilizes the magnetic properties of a ferromagnetic core for control of the magnetic field in critical current density regions of the winding has not been disclosed by prior art devices. The known way to construct a superconductive field winding for a ferromagnetic core of the rotor of a dynamoelectric machine is to connect all portions of the winding in series with all conductors carrying the same current density. The magnetic field produced by the series connected winding arrangement is not uniform and the excitation current in the winding may be increased only until current flow in the point, or points at which the field is greatest reaches the critical value for the disruption of superconductivity. It is a principal object of the present invention, therefore, to provide a superconductive direct current field winding structure which is suitable for a rotor of a dynamoelectric machine which utilizes a ferromagnetic core such that the magnetic properties of the ferromagnetic core may be used to reduce the magnetic field which acts at certain critical points within the winding thereby allowing a larger current to flow in the winding without destroying superconductivity.